The present invention relates to an internal combustion engine waste heat recovery system that includes at least three stages of heat exchangers in an exhaust passage, a working medium flowing through the heat exchangers and carrying out heat exchange with an exhaust gas, and the heat exchanger disposed on the most upstream side of the flow of exhaust gas being positioned immediately downstream from an exhaust valve.
Japanese Utility Model Registration Application Laid-open No. 59-174308 discloses a Rankine cycle system that includes an evaporator for heating a liquid phase working medium with an exhaust gas of an internal combustion engine and generating a vapor, an expander that is driven by the vapor generated in the evaporator, a condenser for cooling the vapor that has passed through the expander and turning it back into the liquid phase working medium, and a feed pump for pressurizing the liquid phase working medium from the condenser and feeding it to the evaporator. In this conventional system, water as the liquid phase working medium is heated by passing it not only through the interior of the evaporator provided in an exhaust pipe of the internal combustion engine but also through the interior of cooling passages formed in a cylinder head and a cylinder block, thereby utilizing waste heat of the internal combustion engine more effectively and cooling the cylinder head and the cylinder block with the liquid phase working medium, so that use of a conventional radiator can be eliminated.
In the above-mentioned conventional system, since the working medium that has increased temperature after passing through the cooling passages of the cylinder head and the cylinder block is supplied to the evaporator, there is only a small difference in temperature between the working medium and the exhaust gas, the working medium passing through the evaporator cannot fully recover the thermal energy from the exhaust gas, the exhaust gas still having recoverable thermal energy is discharged wastefully, and there is a possibility that the entire waste heat recovery efficiency of the internal combustion engine might decrease.
In order to maximize the amount of thermal energy recovered from the exhaust gas, it is desirable to employ a cross-flow system in which the exhaust gas and the working medium flow in opposite directions from each other. That is, since the temperature of the exhaust gas decreases as heat exchange progresses and the temperature of the working medium increases as heat exchange progresses, making the working medium flow within the evaporator from the downstream side to the upstream side of the flow of exhaust gas can maintain a large difference in temperature between the working medium and the exhaust gas throughout the evaporator, thus maximizing the heat exchange efficiency. However, in this arrangement since the working medium has an increased temperature when it reaches a position immediately downstream from an exhaust valve, which is a high temperature section of the exhaust passage of an internal combustion engine, there is a possibility that the high temperature section might not be cooled sufficiently.
The present invention has been accomplished in view of the above-mentioned circumstances, and it is an object of the present invention to cool effectively a high temperature exhaust passage immediately downstream from an exhaust valve while maintaining the efficiency of waste heat recovery from an exhaust gas of an internal combustion engine.
In order to accomplish this object, in accordance with an aspect of the present invention, there is proposed an internal combustion engine waste heat recovery system that includes at least three stages of heat exchangers in an exhaust passage, a working medium flowing through the heat exchangers and carrying out heat exchange with an exhaust gas, and the heat exchanger disposed on the most upstream side of the flow of exhaust gas being positioned immediately downstream from an exhaust valve, characterized in that the working medium is firstly supplied to the heat exchanger disposed on the most downstream side of the flow of exhaust gas, and then supplied to the heat exchanger disposed on the most upstream side of the flow of exhaust gas.
In accordance with this arrangement, with regard to the internal combustion engine that includes at least three stages of heat exchangers in the exhaust passage, since the working medium is firstly supplied to the heat exchanger disposed on the most downstream side of the flow of exhaust gas and then supplied to the heat exchanger disposed on the most upstream side of the flow of exhaust gas, the working medium having a comparatively low temperature can be supplied to the heat exchanger disposed on the most upstream side of the flow of exhaust gas, thus effectively cooling a high temperature section immediately downstream from an exhaust valve of the internal combustion engine and thereby enhancing the durability of the exhaust passage and its peripheral devices, which are exposed to high temperature. Furthermore, since the working medium having the lowest temperature is supplied to the heat exchanger disposed on the most downstream side of the flow of exhaust gas, to which the exhaust gas having a comparatively low temperature is supplied, a difference in temperature between the exhaust gas and the working medium can be maintained and recoverable thermal energy of the exhaust gas can be recovered without waste, thereby increasing the heat exchange efficiency. As a result, the high temperature exhaust passage immediately downstream from the exhaust valve can be cooled effectively while maintaining the efficiency of waste heat recovery from the exhaust gas of the internal combustion engine.
A first stage heat exchanger H1 of an embodiment corresponds to the heat exchanger disposed on the most downstream side of the flow of exhaust gas of the present invention, and a second stage heat exchanger H2 of the embodiment corresponds to the heat exchanger disposed on the most upstream side of the flow of exhaust gas of the present invention.